How to calibrate a digital thermometer?

How to calibrate infrared thermometer?
How to calibrate a digital thermometer?

A digital thermometer should always return accurate readings. Whether you use it for cooking, for measuring body temperature, atmospheric temperature, or any other relevant use, a thermometer should always be made to provide the correct temperature. From time to time, digital thermometers will require recalibrating. This is, fortunately, a very simple task. Here are some easy steps to follow.

How do you calibrate a digital thermometer?

For all thermometers:

• Non-adjustable digital thermometers may be accuracy checked, even if they can’t be adjusted by the user.
• Use a National Institute of Standards and Technology (NIST) certified thermometer for testing thermometer accuracy. (NIST provides certification that a thermometer is accurate within certain limits for a specified time period.)
• Insert the probe of the NIST thermometer into the same ice water or boiling water bath as the test thermometer, making sure they don’t touch each other or touch the bottom or sides of the container.
• Compare readings on both thermometers. Temperature readings using the test thermometer must be adjusted by the degree difference between the NIST and test thermometer. For example, if the NIST thermometer reads the boiling point at 212°F and the test thermometer reads the boiling point at 210°F, all high-range temperature readings using the test thermometer would be adjusted up by 2°F.
• NIST-certified thermometers may be used for accuracy comparison for adjustable digital thermometers and analog thermometers, as well.

How to calibrate a digital food thermometer

Using a food thermometer is the only sure way of knowing your food is at the required temperature— whether you are cooking, cooling, reheating, or holding.

Foodservice workers need to make sure that thermometers measure food temperature accurately, and if not, adjust or replace the thermometer.

This process is called calibration. Thermometers must be calibrated daily to ensure accurate temperature readings. They must also be calibrated after an extreme temperature change (e.g., after measuring a hot food first and then a frozen food, or after a thermometer is dropped). All calibrated thermometers should be accurate within 2 degrees (plus or minus 2 degrees).

Types of food thermometers

The most common types of food thermometers are digital stem and dial stem.

Digital stem thermometers: Digital stem thermometers are preferred because they measure temperature quickly and can be used for thin and thick foods (e.g., a thick piece of meat or a shallow pan containing sauce).

Dial stem thermometers: Dial stem thermometers take longer to measure temperature and must be inserted deeper into food (up to sensory dimple), making them unsuitable for measuring thin foods.

Thermometers should be calibrated using either the ice water or boiling point method. The ice water method is more common and should be used unless ice is unavailable. Note that most digital stem thermometers cannot be calibrated and must be returned to the manufacturer for calibration.

Regardless, you should routinely check to see whether your thermometer is reading temperatures accurately by using one of these methods.

Digital thermometer calibration procedure

One of the critical factors in controlling pathogens in food is controlling temperatures. Disease-causing microbes such as bacteria grow very slowly at low temperatures, multiply rapidly in mid-range temperatures, and are killed at high temperatures. For safety, perishable foods must be held at proper cold temperatures to inhibit bacterial growth or cooked to temperatures high enough to kill harmful microbes.

It is essential to use a food thermometer when cooking meat, poultry, and egg products to prevent undercooking, and consequently, prevent foodborne illness. Many food handlers believe that visible indicators, such as colour changes, can be used to determine if foods are cooked to a point where pathogens are killed. However, recent research has shown that colour and texture indicators are unreliable.

There are two ways to check the accuracy of a food thermometer: using ice water or using boiling water. Many food thermometers have a calibration nut under the dial that can be adjusted. Check the package for instructions.

Boiling Point Method

Bring a deep container of water to a full rolling boil.

Submerge thermometer stem (including the sensory dimple of a dial stem thermometer) for about 30 seconds— until the indicator stops moving. Make sure that the stem does not touch the side or bottom of the container.

• Check to see that the temperature is 212 F (100 C).
  • If the thermometer does not read 212 F (100 C) and the thermometer has a calibration nut: Do not remove the stem from the container and turn the nut until the thermometer reads 212 F (100 C).
  • If the thermometer does not read 212 F (100 C) and the thermometer does not have a calibration nut: Either push the reset button or replace the battery and retest or replace the thermometer

To calibrate an adjustable digital thermometer:

• Ice-Point Method
  • Fill a glass with crushed ice and add cold water until the glass is full.
  • Insert thermometer probe in the center of the glass of ice water, not touching the thermometer to the bottom or sides of the glass.
  • Stir slightly, then wait until the temperature indicator on the thermometer is stabilized.
  • The temperature should read 32°F (0°C). If it doesn’t, push the reset button and adjust to reading 32°F (0°C) while the probe is in an ice water bath.

Ice Water Method

Create a 50-50 ice and water mixture by filling a deep container with crushed ice, adding clean water to fill the container, and stirring well. Submerge thermometer stem (including the sensory dimple of a dial stem thermometer) for about 30 seconds – until the indicator stops moving. Make sure that the stem does not touch the side or bottom of the container.

• Check to see that the temperature is 32 F (0 C).
  • If the thermometer does not read 32 F (0 C) and the thermometer has a calibration nut: Do not remove the stem from the container and turn the nut until the thermometer reads 32 F (0 C).
  • If the thermometer does not read 32 F (0 C) and the thermometer does not have a calibration nut: Either push the reset button or replace the battery and retest or replace the thermometer.

To calibrate an adjustable digital thermometer:

• Ice-Point Method
  • Fill a glass with crushed ice and add cold water until the glass is full.
  • Insert thermometer probe in the center of the glass of ice water, not touching the thermometer to the bottom or sides of the glass.
  • Stir slightly, then wait until the temperature indicator on the thermometer is stabilized.
  • The temperature should read 32°F (0°C). If it doesn’t, push the reset button and adjust to reading 32°F (0°C) while the probe is in an ice water bath.

Properly Calibrating an Infrared Thermometer

Infrared thermometers can be calibrated for accuracy just like other thermometers. In calibration labs, technicians use industrial black bodies to calibrate infrared thermometers.

Black bodies approximate zero reflected ambient radiation and therefore the unimpeded emission of infrared energy for a given emissivity value.

Short of using a black body, the next best way to calibrate your infrared thermometer is to use a simple, inexpensive Infrared Comparator Cup. It is best to calibrate your infrared thermometer at more than one temperature and the Infrared Comparator Cup makes it easy.

If neither an industrial black body or a comparator cup are available, however, you can do a quick calibration using a properly made ice bath.

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